Bring a large pot of salted water to a boil. Add the pasta and cook until al dente, approximately 8-10 minutes. Reserve 1 cup of pasta water and drain the rest.

Finely dice the onion, mince the garlic, and grate the parmesan cheese to prep ingredients.

Dry the salmon and season with salt and pepper. In a skillet, heat olive oil over medium-high heat. Cook the salmon for 3-4 minutes on one side until golden, then flip and cook for 2-3 minutes more.

In the same skillet, reduce heat to medium and sauté the onions for 3-4 minutes until translucent. Add the butter, garlic, and parsley, then sauté for another 30 seconds.

Add 20% of the cooked salmon and red pepper flakes to the skillet. Pour in the lukewarm milk while stirring to avoid lumps, and let simmer for a minute.

Add the reserved pasta water and parmesan cheese to the skillet, stirring to create a silky sauce. Simmer for a minute, adjusting the thickness with more pasta water as needed.

Fold the cooked pasta into the sauce, adding the remaining salmon and warm through for about a minute.

Serve in warm bowls, garnished with lemon zest, extra parmesan, and red pepper flakes.